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A rocky object in orbit around the sun.

Apollo missions     NASA’s third human spaceflight program eventually took humans to the lunar surface. Along the way, this program sought to develop the technologies needed for long-distance space travel. It got a big kick-start after President John F. Kennedy proposed in 1961 creating the national goal of “landing a man on the Moon and returning him safely to the Earth.”

asteroid     A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

atmosphere     The envelope of gases surrounding Earth or another planet.

comet     A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”

density     The measure of how condensed some object is, found by dividing its mass by its volume.

dwarf planet     One of the solar system’s small celestial objects. Like a true planet, it orbits the sun. However, dwarf planets are too small to qualify as true planets. Prime examples of these objects: Pluto and Ceres.

electron microscope     A microscope with high resolution and magnification that uses electrons rather than light to image an object.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

Jupiter     (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

Kuiper belt     An area of the solar system beyond the orbit of Neptune. It is a vast area containing leftovers from the formation of the solar system that continue to orbit the sun. Many objects in the Kuiper belt are made of ice, rock, frozen methane and ammonia. The best known of the larger Kuiper Belt objects is the dwarf planet Pluto. The belt extends out from the sun at a distance of 30 to 55 astronomical units. (An astronomical unit is equal to Earth’s from the sun.)